Gimbaled mirror systems are used to steer a line of sight for pointing and tracking in optical sensor systems. The system controls the orientation of a tracking mirror which reflects the incoming light from a variable line of sight (LOS) into a fixed input (in position and angle) of a collection of optical sensor equipment referred to as an “optical sensor suite”. Common configurations place the fixed sensor suite input along a vertical or horizontal direction. In a mobile platform such as an aircraft, for example, the mirror system may be mounted adjacent to a window of an outer wall of the aircraft.
For a traditional gimbaled-mirror tracking system, there is a tradeoff among the following aspects of the system:                Field of regard (the angular view range forward, back, up, down)        Aperture (beam size)        Window size        
It is generally desirable for field of regard and aperture to be as large as possible, and for the window size to be as small as possible. The field of regard (FOR) is the total set of look-angles the system can see. A larger FOR gives a more capable and flexible system, with fewer operational constraints. A larger aperture gives a stronger optical signal and greater resolution. A smaller window makes integration of the tracking system simpler and less expensive.
In many cases, there are hard constraints on window size. For example, windows may need to fit between structural aircraft ring frames. Some optical materials are not available in large panes. At a minimum, large windows are more expensive, often prohibitively so. Some systems have an uncovered opening or cavity instead of a window, and in such systems the opening should be as small as possible for reasons of aerodynamics.
It has been known in the prior art to use a translation stage to move the entire optical system as part of the tracking operation. Such a translation stage may be very large, because the overall optical system will often weight thousands of pounds. It may also be difficult in such a system to coordinate multiple sensor channels, because each must be on a separate translation stage, or their relative motion must be coordinated.